Can thread adaptors or reducers be used on an Ex d cable entry?
Abstract
Yes, if the adaptor is itself certified flameproof for the gas group and its threads match the enclosure entry and the gland. For adaptors fitted on site, no more than one per entry, and never an adaptor combined with a stopping plug.
Yes, a thread adaptor or reducer may be fitted to the entry of a flameproof (Ex d) enclosure, provided it is itself certified for flameproof use and the right gas group, and its threads match both the enclosure entry and the gland. IEC 60079-1 limits each entry to one adaptor and forbids combining an adaptor with a blanking element (stopping plug), so adaptors cannot be stacked and an unused entry is closed with a plug fitted directly into the enclosure thread. These limits apply to separately certified adaptors that are fitted on site.
Short answer
- Use only adaptors and reducers certified as flameproof (
Ex db) for at least the gas group and temperature range of the enclosure, and keep to their specific conditions of use (the "X" on the certificate). - One adaptor per entry. Stacking a reducer on an adaptor on another reducer is not allowed.
- No adaptor with a stopping plug. Close a spare entry with a certified blanking element screwed straight into the enclosure.
- The thread type and size on each side must match exactly (for example M20 x 1.5 into an M25 entry needs an M25 to M20 reducer, not a "near enough" thread), with full engagement.
- The enclosure documentation decides which entries exist and what may go into them; an adaptor never makes a new entry.
- Exception: an adaptor fitted by the manufacturer and assessed as part of the certified enclosure is treated as part of the enclosure, and IEC interpretation sheet ISH1:2020 to IEC 60079-1:2014 allows a blanking element in it.
Why the adaptor rules exist
A threaded entry is a flamepath. The flame from an internal explosion must cool along the engaged threads before it reaches the outside, which is why IEC 60079-1 fixes a minimum number of engaged threads and a minimum depth of engagement for threaded joints. An adaptor adds a second threaded flamepath in series with the first, and a reducer that is badly made or loosely fitted can leave either joint with too few threads in contact.
Stacking adaptors multiplies the joints and the leverage on the first thread, so a knock on the gland can loosen the entry. Combining an adaptor with a stopping plug gives a closure made of two removable parts, either of which could be undone or loosened without anyone noticing that the enclosure is open. These are the practical reasons behind two plain rules in IEC 60079-1: each entry may carry no more than one thread adaptor, and a blanking element must not be used with an adaptor. The installation standard IEC 60079-14 has its own clause on adapters and blanking elements for the installer.
An interpretation sheet issued in 2020 (ISH1 to IEC 60079-1:2014) clarifies that these two rules concern separately certified Ex equipment thread adaptors. An adaptor fitted in the factory and assessed with the enclosure may even carry a blanking element, and the thread marking then refers to the adaptor's field-wiring thread. The certificate shows which case applies.
What to check before fitting an adaptor
| Check | What to look for |
|---|---|
| Certificate of the adaptor | Flameproof type of protection, gas group at least that of the enclosure, ambient range, any X conditions |
| Enclosure documentation | Entry size and thread type (marked beside the hole, on the nameplate or in the instructions) and the permitted number of entries |
| Thread match | Same thread system both sides: metric to metric, NPT to NPT, or a certified adaptor between systems |
| Engagement | Full engagement of the thread form; a sealing washer where the instructions require one for the IP rating |
| Combination | Only one adaptor per entry; no stopping plug in an adaptor |
| Gland selection | The gland must still suit the cable, the zone and the IEC 60079-14 selection rules; the adaptor does not change that |
IEC 60079-1 requires each threaded entry hole to be identified by thread type and size, for example M25 or 1/2 NPT. That marking is the starting point: if it says 3/4 NPT, a metric gland needs a certified 3/4 NPT to M20 (or M25) adaptor, and an M20 gland screwed into a 3/4 NPT hole is wrong even if it seems to bite.
For external metric threads on glands and adaptors, the 2014 edition of IEC 60079-1 asks for a threaded length of at least 8 mm with at least eight full threads, so that at least five threads are engaged once installed. NPT threads follow their own table. The 2014 edition also allows only metric and NPT threads for entries into the enclosure itself, while adaptors, glands and blanking elements may use other thread forms. Check the edition named on your equipment certificates: IEC 60079-1:2014 has been replaced by IEC 60079-1:2026.
Practical case: an M25 entry and an M20 gland
A Zone 1 junction box marked Ex db IIB T6 Gb has M25 entries. The cable fits an M20 barrier gland.
- Fit a reducer M25 to M20 that is certified
Ex dbfor IIB or IIC, and check its ambient range covers the site. - Screw it fully into the M25 entry with any sealing washer the instructions call for, then fit the M20 gland directly into the reducer.
- Do not add a second adaptor to reach a 1/2 NPT gland that happens to be in stock.
- Any unused M25 entries are closed with certified M25 stopping plugs, not with M20 plugs in reducers.
- Record the adaptor certificate in the installation documentation and the inspection records.
Common mistakes
- Adaptor plus stopping plug to close a spare entry, often left from an earlier cable change.
- Two or three adaptors in a row to bridge between thread sizes.
- Mixing NPT and metric threads without an adaptor, so only a few threads engage.
- Using an adaptor certified only for increased safety (Ex e) on a flameproof enclosure.
- Re-tapping or enlarging the entry to avoid the adaptor; that changes the certified enclosure.
Related on excodex
- Flameproof enclosures (Ex d): how flamepaths stop an internal explosion
- Cable glands for hazardous areas: barrier glands, compound glands and selection
- Do unused cable entries in an Ex d enclosure need certified stopping plugs?
- Can you drill an extra cable entry hole in an Ex d enclosure on site?
- Standards spaces: IEC 60079-1, IEC 60079-14
Standards and further reading
- IEC 60079-1, equipment protection by flameproof enclosures "d" (threaded joints, entries, documentation on adaptors and blanking elements).
- IEC 60079-14, design, selection and installation of electrical installations (adapters and blanking elements, cable entry devices for Ex d).
- IEC 60079-1:2014 text with ISH1:2020 (thread adaptors and blanking elements), incorporated by reference in US federal regulations; replaced by IEC 60079-1:2026: law.resource.org
- IEC 60079-14:2024 preview (table of contents, including the clause on adapters and blanking elements): standards.iteh.ai
- MSHA comparison of IEC 60079-1 and US mining requirements (archived), threaded joints and cable entries: webarchive.library.unt.edu
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.